1. Field of the Invention
The present invention relates to an olefin type thermoplastic elastomer powder for powder molding and a powder molding method using the same and a molded article thereof. More specifically, it relates to a thermoplastic elastomer powder for a powder molding process such as a powder rotational molding, and suitable for use, in particular, as covering materials and housings for molded articles in various fields, regardless of the backing material used, a powder molding method using the same and a molded article thereof.
2. Description of the Related Art
With regard to covering materials used as interior decorative materials of automobiles, there is an increasing demand for those which have a light weight and a good and soft feeling, and which can be given an embossed pattern or stitch pattern to increase the value of molded articles. Also, during the scrapping of cars, acidic substances are generated by a combustion of such interior decorative materials to cause a contamination of the air, and thus create serious social problems such as acid rain, etc., and therefore, there is a strong demand for non-polluting materials.
Known covering materials of the prior art are vacuum molded olefin type thermoplastic elastomer (hereinafter called TPO) sheets and vacuum formed sheets composed mainly of vinyl chloride resins and ABS resins, or rotation molded or slush molded sols composed mainly of vinyl chloride resins for paste and plasticizers (hereinafter called sol molding).
Vacuum formed TPO sheets attain the objects to provide a light weight and non-polluting material, but it is difficult to impart complicated shapes thereto.
Also, vacuum formed sheets contain a high residual molding stress caused during the forming process, and thus have a drawback in that cracks will appear after long term usage.
Vacuum formed sheets composed mainly of vinyl chloride resins and ABS resins have drawbacks similar to those of vacuum formed TPO sheets, in that it is difficult to impart complicated shapes thereto, and further that, compared with TPO sheets, they have the drawback of heavier weight and cause pollution.
Sol moldings composed mainly of vinyl chloride resins for paste and plasticizers give a soft feeling, and complicated shapes can be imparted thereto. However since the gellation temperature is low, they meltrapidly while being molded, and thus many drawbacks arise, for example, problems in the processing such as flow marks, lip or the sol fiber-forming phenomenon, the inherent problems of vinyl chloride such as a heavier weight and pollution, and further problems in that the inner window glass of the automobiles produced from said sol molding suffers from hazy appearance due to the use of a large amount of plasticizers.
Due to these drawbacks and problems of the known molding methods, powder molding methods have recently attracted attention.
Powder molding methods include, in general, a flow dipping method, an electrostatic coating method, a flame spray coating method, and a powder rotational molding method, and particularly for producing interior decorative materials for automobiles, the powder rotational molding method is suitable.
A partially crosslinked TPO composition is known from Japanese Unexamined Patent Publications (Kokai) Nos. 48-26838, 53-149240, but the known molding methods currently used for the partially crosslinked TPO are:
a) injection molding (shear rate during processing: .gamma..gtoreq.10.sup.3 sec.sup.-1); PA1 b) extrusion molding (10.sup.1 .ltoreq..gamma..ltoreq.10.sup.2 sec.sup.- 1); PA1 c) calendering (10.sup.2 .ltoreq..gamma..ltoreq.10.sup.3 sec.sup.-1); PA1 d) compression molding of the primarily processed product in b. or c.; and PA1 e) vacuum forming of the primarily processed product in b. or c.,
but all of these methods require the molding temperatures are higher or equal to the softening point, and the molding pressures must be varied depending on the viscosities and the shear rates corresponding to the respective processing conditions.
Nevertheless, in the molding method such as a powder molding at a shear rate of 10.sup.0 sec.sup.-1 or less or under an approximately stationary state of the polymer, at a vibration frequency of 1 radian/sec., and with no application of a pressure or under a very low pressure (.ltoreq.1 kg/cm.sup.2), the flowability becomes extremely poor, and accordingly, the molding becomes very difficult. Further, even if a molding is possible, the poor flowability in the low shear rate region cause an incomplete thermal fusion between the powder particles, and only molded articles with a low mechanical strength can be obtained.
For this reason, for example, Japanese Unexamined Patent Publication (Kokai) No. 2-57309 discloses the following inventions:
"1) An embossed thermoplastic elastomer molding, having an emboss pattern formed on the surface by melting a thermoplastic elastomer powder comprising a polyolefin resin and an ethylene-.alpha.-olefin copolymer rubber into a flowing plasticized state in a roating and heated mold with an emboss pattern.
2) A method of preparing a thermoplastic elastomer molding, which comprises introducing a thermoplastic elastomer powder comprising a polyolefin resin and an ethylene-.alpha.-olefin copolymer rubber into a mold with an emboss pattern, followed by sealing the mold, then melting the thermoplastic elastomer powder by roating and heating the mold into a flowing plasticized state, and subsequently cooling the mold to obtain a thermoplastic elastomer molding having an emboss pattern on the surface." Namely, this is an example of how to obtain a TPO molding by a powder rotational molding, in which TPO with a low flowability in the low shear rate region is melted and adhered, while in a flowing plasticized state, to the inner surface of the mold which is rotated and heated.
According to this method, however, the flowability of TPO powder in the low shear rate region is still poor, and therefore, the thermal fusion strength between powder particles is too low, causing a problem that pinholes, etc. may be formed, and a molding having a fully satisfactory appearance and physical properties, etc., has not been obtained.